1. Field of the Invention
This invention pertains generally to methods and devices for calculating decompression schedules for underwater divers.
2. Description of the Prior Art
The problem of decompression sickness, or the "bends", is a well-known phenomena observed in divers who surface after spending substantial periods of time under water. Decompression sickness is caused by the so-called "inert" gas component of the diver's breathing mixture, such as nitrogen in a normal air mixture. As the diver descends, the pressure of the breathing mixture in the diver's lungs must necessarily be increased, and the inert gases in the breathing mixture tend to slowly absorb into the body fluids and tissues of the diver at a rate which depends in part upon the pressure of the breathing mixture. As the diver ascends, the inert gases absorbed by his fluids and tissues are released therefrom and are ultimately discharged from the diver's body through his lungs.
Although the physiological mechanism of decompression sickness is not completely understood, a too rapid release of the pressure on the diver's body will apparently cause the absorbed inert gases to form bubbles within the tissues of the diver which are of sufficient magnitude to cause damage to the body tissues. It has been observed, however, that rapid changes in pressure on the diver's body which do not exceed certain maximum pressure changes will not result in the onset of decompression sickness. It has been found that the ratio of the absolute pressure of the inert gases within the body tissue with respect to the ambient pressure on the body of the diver must not exceed a certain maximum ratio, generally called the supersaturation ratio, if decompression sickness is to be avoided. It has also been found that the supersaturation ratio varies as a function of the absolute tissue pressure.
Since the human body has many different types of tissues, it may be expected that the various tissues in the body would have different supersaturation ratios and different rates at which inert gases are absorbed and eliminated by the tissues. An early model of the actions of the body tissues was proposed by Boycott, Damant and Haldane, "The Prevention of Compressed--Air Illness," J. Hygiene, Vol. 8, pp. 342 et seq. (1908), which analogized the human body to a finite number of gas diffusion chambers pneumatically connected in parallel, with each chamber having a different supersaturation ratio and a different time constant of diffusion.
The decompression tables utilized by the United States Navy are substantially based on the theory introduced by Boycott et al. However, other models of the physiological behavior of body tissues under pressure have been developed, and various computational devices have been employed to simulate the body functions based on these models. Typically, such calculators have utilized several body tissue analogs having different time constants, as for example, a plurality of chambers wherein gas under pressure diffuses through a membrane in the compartments. Other calculators have been developed which utilize an electrical analog of such gas diffusion. It is apparent that with such multicompartment models it is necessary to continuously monitor all compartments to determine the highest pressure compartment in order to calculate a safe decompression stop. Such calculators have thus been complicated and are generally expensive.
Most decompression calculators such as those described above are based on physiological models which assume that the time constant of absorption and the time constant of elimination of gas from a tissue are the same. This is not a valid assumption, as demonstrated by H. V. Hempleman, "The Unequal Rates of Uptake and Elimination of Tissue Nitrogen Gas in Diving Procedures," Medical Research Counsel, R. N. Personnel Research Committee, U.P.S., pp. 195 et seq., (1960). The complexity required of the multiple compartment decompression plan calculators, or their electrical equivalents, also makes it virtually impossible to account for the differences in supersaturation ratio and tissue time constants which occur from individual to individual.
Various empirically derived tables have been developed by the Navies of the United States, Canada, and other countries. These tables were prepared by testing with subject divers to determine maximum rates of decompression without the onset of decompression sickness. While these tables are useful, they do not have sufficient data to plan dives which vary in time and depth from the dive plans used in preparing the tables. It may also be noted that the decompression tables of the various Navies do not agree uniformly. For example, the tables of the Canadian Navy prescribe a more conservative (longer duration) decompression schedule than do the U.S. Navy tables for dives of relatively short duration.